1. Field of the Invention
This disclosure relates to servicing a wellbore. More specifically, it relates to servicing a wellbore with sealant compositions comprising a calcium aluminate cement and a set modifier and methods of using same.
2. Background of the Invention
Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. The drilling fluid is then usually circulated downward through the interior of the pipe and upward through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed.
For cementing high temperature wells, for example steam injection wells or steam production wells, calcium aluminate cements (CACs) are typically used. In such wells, the higher temperature resistance of calcium alumina cements compared to Portland Cement/silica mixtures is an advantage for long term integrity of the cement sheath. Additionally, for cementing high temperature wells, especially those containing carbon dioxide, for example geothermal wells or carbon dioxide injection wells, calcium aluminate cements (CACs) are typically employed because the use of conventional hydraulic cement compositions may result in well failure. At high static subterranean temperatures, and in the presence of brines containing carbon dioxide, conventional hydraulic cements rapidly deteriorate due to alkali carbonation. CACs combined with a soluble phosphate salt, for example sodium metaphosphate and a filler such as Class F flyash, are commonly used in primary and secondary high temperature cementing operations to form quick setting, strong cement that upon setting binds well to the subterranean formation and to itself, resists carbonation, has high strength, and low permeability. These compositions are described in Journal of Material Science, 32, 3523-3534 (1997) and J. Material Science, 37, 3163-3173 by Sugama et al, and in U.S. Pat. Nos. 5,900,053, 6,143,069 and 6,332,921 issued to the assignee of the current invention, each of which is incorporated by reference herein in its entirety. One drawback to the use of CACs at higher temperatures is that the cementitious compositions set very rapidly, e.g., within a few minutes at elevated temperatures. The rate of reaction further increases as the temperature increases. As such, the thickening times of the compositions may be unacceptably short to allow them to be pumped to their desired downhole locations, making the use of such compositions in well cementing a challenge. For example, the drill pipe or the tool used to lower the piping in the wellbore may be cemented in place, causing delay in the completion of the wellbore.
One method commonly employed to lengthen the thickening time of CAC compositions is to introduce set retarders into the compositions, thereby delaying the time to setting of the cement. As a result, the thickening time of the CACs may endure for hours when used in a wellbore having a relatively high bottom hole static temperature. Organic acids such as citric acid or tartaric acid are examples of typical set retarders that may be used with the CAC. While these organic acids may allow the cementitious composition to remain pumpable for an extended time period these acids are costly additives and negatively impact the overall economics. A need therefore exists for inexpensive materials that increase the thickening time of CAC compositions when they are exposed to relatively high downhole temperatures and pressures.